A transport vehicle for perishable goods is, by definition, an insulated storage chamber where products are maintained at a predetermined temperature during the transportation by means of a heating/cooling system. For a good performance, the heating/cooling system must have the ability to quickly absorb heat or cold which could penetrate the chamber, as well as the heat generated by certain type of products, in order to maintain the product in a satisfactory condition.
Mechanical refrigeration is the standard for refrigerated transport vehicles eventhough it has been widely recognized that this temperature control approach is quite complex and does not guarantee the basic requirements outlined above.
In an attempt to upgrade the dependability and the cooling capability of refrigerated transport vehicles and avoid dessication problems as well as unadequate air circulation associated with mechanical refrigeration, cryogenic cooling systems have been developed during the past recent years, theoretically outperforming by far mechanical refrigeration units.
Cryogenic cooling has evolved along two different approaches to solve the problem of temperature control. The injection approach is the simplest one. It consists of spraying liquid cryogen, such as CO.sub.2 kept under high pressure and low temperature, directly into the insulated enclosure at atmospheric pressure. Immediately, dry snow (solid CO.sub.2) and CO.sub.2 vapors at -110.degree. F. are formed. As the dry snow sublimates, it absorbs heat at the rate of 246 Btu per pound of snow, or at 120 Btu per pound of liquid CO.sub.2 injected.
The major drawback of the direct injection method resides in that it does not guarantee a uniform temperature. The injected CO.sub.2 can reduce the temperature of the enclosure in an uncontrolled manner and so rapidly to a point where damage to the stored product may occur, especially when the CO.sub.2 comes in direct contact with a sensitive product. In addition, the CO.sub.2 rarefies the oxygen in the enclosure causing problems to human beings and to some breathing products. For all these reasons, the injection technique is mostly used for freezing where precise temperature control is not really essential.
The other approach, usually referred to as "vaporization" consists in recuperating the latent heat obtained when liquid CO.sub.2 converts to gas inside an evaporator. This method allows a more precise temperature control and does not affect the oxygen content of the atmosphere in the storage chamber.
Cooling systems, based on the vaporization approach, are definitely an improvement over cryogen injection units in terms of temperature control, however they are still far from being fully satisfactory in this respect.